Really, there's no definable way to tell when your black plates were issued by the letters. The problem is that when California DMV's were issued black plates, large metropolitan areas went through theirs fairly quickly while rural areas were much slower. For instance, if you licensed your new 69 Camaro in Los Angeles, you would have probably received a set of plates starting with the letter "Y". Los Angeles registered hundreds of cars a day, so they went through the alphabet fairly quickly. However, if you licensed your new 69 Camaro in say, Birds Landing (California's smallest town), you would have probably received plates starting with the letter "B" or "C"! Small rural towns didn't register as many cars as the large cities, so they didn't use up their allotment as quickly. See what I mean?
I've seen original late 60's cars with original black plates, and the letters were only in the "E" and "F" ranges! No way to tell for sure...
As for YOM, here's the official text direct from the California DMV:
14.100 Year of Manufacture Model Year Date Plates (VC §5004.1)
Year of Manufacture (YOM) license plates are any officially manufactured California
license plates issued prior to 1963 that are determined to be legible and serviceable by
the DMV. YOM plates may be assigned in lieu of regular license plates to any
currently registered 1962 or older year model motor vehicle or trailer of a
corresponding model year.
Kit cars of vintage automobiles (a replica of a 1928 vehicle for example) do not
qualify for YOM plates. Vehicles with YOM plates have full operating privileges and
are not restricted to participation in historical vehicle activities.
The plates must be supplied by the vehicle owner and authenticated by DMV
personnel. Base year plates must have the appropriate year validation sticker attached
to the plate to be authenticated as other than the base year (for example, a 1956 plate
with a 1958 sticker equals a 1958 plate).